1. Field of the Invention
The present invention relates to a process for preparing a thin film of superconductor. More particularly, it relates to a process for depositing on a substrate a superconducting thin film of a compound oxide having a higher transition temperature of superconductivity which can be used for Josephson Junctions devices or the like.
More particularly, the present invention relates to a process for depositing a superconducting thin film of K.sub.2 NiF.sub.4 -type oxides such as [La, Ba].sub.2 CuO.sub.4, [La, Sr].sub.2 CuO or the like.
2. Description of the Related Art
Superconductivity is a phenomenon in which the electrical resistance becomes zero and hence can be utilized to realize a variety of devices and apparatus which are required to reduce consumption of electrical energy and several ideas of its applications which utilize the phenomenon of superconductivity have been proposed. However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the highest recorded Tc in the case of Nb.sub.3 Ge did not exceed 23.2 K.
It is clear that liquefied helium (boiling point of 4.2 K) is the only known cryogen which can realize the very low temperature of Tc. However, helium is not only a limited and costly resource, but also requires a large-scaled system for liquefaction. Therefore, other superconducting materials having a higher Tc had been desired. However, no material which exceeded the abovementioned Tc had been found during all of the studies conducted for the past ten years.
The possibility of the existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64 (1986) 189).
These new oxide type superconducting materials are [La, Ba].sub.2 CuO.sub.4 and [La, Sr].sub.2 CuO.sub.4, which are referred to as the K.sub.2 NiF.sub.4 -type oxides having a crystal structure similar to Perovskite-type superconducting oxides which have been known (See for example, U.S. Pat. No. 3,932,315). The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as high as 30 k which are much higher than the known superconducting materials and hence it became possible to use liquified hydrogen (b.p.=20.4 K) or liquified neon (b.p.=27.3 K) as a cryogen which causes them to exhibit superconductivity characteristics. Particularly, hydrogen is an inexhaustable resource with the adverse drawback of including a danger of explosion.
Conventional methods for producing superconducting compounds or metal alloys may be classified into two categories: (a) the powder sintering technique in which starting materials are blended into a mixture which is then sintered, and (b) the vapor deposition technique in which an alloy or a compound is vaporized into the gaseous phase and deposited or grown on a substrate.
FIG. 3 shows a series of operations in the typical powder abovementioned sintering process which includes the following steps:
(1) uniformly mixing powders having a particle size of several .mu.m such as BaCO.sub.3, La.sub.2 CO.sub.3 or SrCO.sub.3, and CuO, PA1 (2) molding the powders in a mold, PA1 (3) performing preliminary sintering of the molded material at 900.degree. C. for 12 hours, PA1 (4) removing the sintered article from the mold and then pulverizing the same again, PA1 (5) press-molding the pulverized power, and PA1 (6) sintering (reaction-sintering) the press-molded material at 1,100.degree. C. for 2 hours.
The vapor deposition technique has been used for producing a thin film of superconducting material such as Nb.sub.3 Ge and BaPb.sub.1-x Bi.sub.x O.sub.3. In the case of Nb.sub.3 Ge, particles of Nb and Ge are sputtered from several targets each consisting of Nb and Ge, irrespectively, and are deposited onto a substrate to form a film composed of Nb.sub.3 Ge. Japanese patent laid-open No. 56-109,824 discloses a process for producing a thin film of BaPb.sub.1-x Bi.sub.x O.sub.3 by means of a sputtering technique.
However, the above mentioned new type superconducting materials of K.sub.2 NiF.sub.4 -type oxides which have just been discovered have been studied and developed only in the form of sintered bodies or as a bulk produced from powders which have been previously press-molded. In other words, heretofore, no studies of film-deposition techniques have been conducted on these new type superconductors. Therefore, the resultant superconducting bodies must have a bulk form which is difficult to be utilized in the form of a thin film which is required in the field of electrical devices or elements.
Therefore, an object of the present invention is to provide a process which can change the K.sub.2 NiF.sub.4 -type superconducting oxides into a thin film form.